"Mark Zacharias" wrote in message
...
"Arfa Daily" wrote in message
...
Anyone read anywhere what the failure mechanism is for solid dielectric
caps of the tantalum variety ? In my experience, no matter what the
value, working voltage, or format (bead, bullet or box), they always seem
to fail leaky. Not open or short (well, very occasionally short). Just
leaky.

Example. Today, I had a Mesa Boogie combo cross my bench. Very odd
problem in that when the 80Hz slider in the graphic was advanced in the
'boost' direction, the audio suddenly went very distorted, and then
disappeared. There was also a slight 'scratchiness' to this pot, which
did not feel like a bad or dirty track.

When I got the graphic pots board out, it was actually quite a simple
affair, with each of the 6 bands having just a pot, one resistor, one
choke, and one cap. On the 80Hz channel, this cap was a 3u3 tantalum
bullet, and it was 2k leaky. Why ? The device is under no voltage stress
at all in this position, being subject to low signal levels only. I
wonder if it's some kind of internal 'growth' like the dreaded tin
whiskers, which causes it ?

A new cap (used a 3u3 tant bead that I had in stock) restored normal
operation of the equaliser, and all scratchiness in that band
disappeared.

Arfa


Tantalums have kind of a bad reputation for shorting, and that's been my
experience. Not really seen them leaky or open.

The scratchy sound of the pot is typical of DC leakage in such a circuit.
Can make a volume pot or similar sound "dirty" or make woofers go "whoomp"
when the control is moved.

But then you know this, yes? I know you have loads of experience with this
stuff.


Indeed yes, Mark. That was what gave the clue that it was going to be the
cap, and caused me to unsolder it, and reach for the meter ... :-)



I got the "whoomp - whoomp" in a Yamaha receiver the other day - a bad
muting transistor put DC on the volume control.

Mark Z.

I fear that such knowledge, that we were taught as young techs by others
much older and wiser than ourselves, is doomed to be conscripted to the
'lost' pile, as we expire one by one ... :-(

Arfa